[Haskell-cafe] SYB with class: Bug in Derive.hs module

[Andrea Vezzosi]

On Mon, Sep 3, 2012 at 12:00 PM, Roman Cheplyaka <roma at ro-che.info> wrote:
> There's a bug in syb-with-class reported by Alexey Rodriguez Yakushev in
> 2008 [1]. I can confirm that the bug is still there (syb-with-class-0.6.1.3,
> ghc 7.4.1).
>
> [1]: http://www.haskell.org/pipermail/haskell-cafe/2008-March/041179.html
>
> Here's an even simpler test case:
>
>     {-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses,
>         UndecidableInstances, TemplateHaskell, OverlappingInstances,
>         DeriveDataTypeable #-}
>     import Data.Generics.SYB.WithClass.Basics
>     import Data.Generics.SYB.WithClass.Derive
>
>     data Foo = Foo Foo | Bar
>       deriving (Typeable, Show)
>
>     deriveData [''Foo]
>
>     f :: (Data NoCtx ast, Typeable ast) => ast -> TypeRep
>     f = typeOf
>
>     main = print $ f $ Foo Bar

This is pretty similar to what ended up being a ghc bug, fixed in 7.0 though:
http://hackage.haskell.org/trac/ghc/ticket/3731

> The cause of this bug is a self-referencing instance created by
> deriveData:
>
>     instance (Data ctx Foo, Sat (ctx Foo)) => Data ctx Foo where ...
>
> What's the proper way to fix it?